CA problem in new 2008 R@ domain.

Six months ago I updated our network with a new Server 2008R2 server intended to replace our older  Server 2003 box. I made the new server a DC and migrated the users over to it. I then made new shares and moved the FSMO roles to the 2008 R2 server. Everything has been working fine. This network also has a child domain at a remote location. I removed the old 2003 DC from the network back 6 months ago. I now realize I should have run dcpromo to demote the old server as I keep getting errors on the new server  the CA can’t find the old server. (event ID 13). I still have the old server here in the computer room. Do I need to hook it back up and run dcpromo and demote and will that fix the CA errors? Can I just generate a new CA for the new server (that has a different name and IP than the old one)? In the last few day some of my users have started to have to append the @domain to the end of their user names in order to log in. Is this related? Thanks in advance for your help.